Abstract Photon upconversion has potential applications in light–emitting diodes, photocatalysis, bio‐imaging, microscopy, 3D printing, and photovoltaics. Bulk lead‐halide perovskites have emerged as promising sensitisers for solid‐state photon upconversion via triplet–triplet annihilation due to their excellent optoelectronic properties. In this system, a perovskite‐sensitiser absorbs photons and subsequently generates triplet excitons in an adjacent emitter material, where triplet–triplet annihilation can occur, allowing for the emission of higher energy photons. However, a major loss pathway in perovskite‐sensitised upconversion is the back‐transfer of singlet excitons from the emitter to the sensitiser via Förster Resonance Energy Transfer. In this investigation, a 2D perovskite spacer layer is introduced between the bulk perovskite‐sensitiser and rubrene emitter to mitigate back‐transfer of singlet excitons. This modification reveals the inherent balance between efficient triplet exciton transfer across the interface with a potential barrier vs the mitigation of near‐field back‐transfer by increasing the distance between the sensitiser and singlet excitons in the emitter. Notably, the introduction of this spacer layer enhances the relative upconversion efficiency at lower excitation power densities while also sustaining performance over extended timescales. This work represents significant progress toward the practical applications of perovskite‐sensitised photon upconversion.

Abstract The selective growth, unambiguous identification, and optical characterization of 2H and 3R polytypes in bulk single crystals of the van der Waals polar insulator α‐In 2 Se 3 are reported. High‐quality single crystals are obtained by optimizing chemical vapor transport (2H) and horizontal Bridgman (3R) methods, and their phases are reliably and efficiently verified by X‐ray Laue back‐reflection. Optical transmission spectra show the 2H phase has a slightly smaller band gap and steeper absorption edge than the 3R phase, consistent with first‐principles predictions of their electronic structures. Furthermore, absolute reflectance measurements unveil rich peak‐valley structures reflecting distinct van Hove singularities in their joint density of states, providing a physical characteristic that more clearly distinguishes the polytypes than the band‐gap feature. The good agreement with DFT simulations confirms that absolute reflectivity provides crucial insights into fine electronic structures beyond the band‐gap region. The results highlight pathways to further explore polytype‐dependent functional properties of α‐In 2 Se 3, such as nonlinear optical responses.

Abstract Gasochromic hydrogen indicators combine easily readable color changes with low‐cost, low‐maintenance operation, making them attractive for reliable hydrogen monitoring in real‐world settings. In this study, gasochromic indicator supraparticles are synthesized from silica and platinum nanoparticles with resazurin as the gasochromic element. These powders, although responsive, are impractical in the field due to handling losses and limited mechanical robustness. Therefore, the supraparticles are implemented into a polymethylmethacrylate (PMMA) matrix to create millimeter‐sized beads, so‐called suprabeads, and self‐supporting thin films. The gasochromic behavior of the powder, suprabeads, and thin films is systematically compared. All formats exhibit the same color transition sequence under hydrogen exposure – purple to pink to colorless – with the pink state recovered in air. The effects of hydrogen concentration, relative humidity, and light exposure on the gasochromic response are thoroughly analyzed, and detailed electron microscopy studies linked structural information to the performance in all application scenarios. No significant differences in the color change mechanism are observed between the different applications; however, increasing the polymer‐matrix thickness slowed the indicators’ response while improving resistance to desiccation. The findings highlight the potential of these materials for hydrogen sensing in real‐world scenarios alongside the hydrogen economy.

Abstract Plasmonic nanostructures photoexcited with ultrashort light pulses exhibit a strong nonlinear optical response driven by nonequilibrium ‘hot’ carriers. Studying the spectro‐temporal evolution of such nonlinearities to extract information on hot electron dynamics has attracted significant interest, given the unparalleled opportunities unlocked by these high‐energy carriers in fields ranging from photocatalysis to optical communications. However, in typical samples of size‐dispersed nanoparticles, effects such as inhomogeneous broadening and pump‐pulse‐induced selectivity can distort the system response, hindering accurate characterizations. This study dissects the ultrafast response of polydisperse gold nanorods employing two‐dimensional electronic spectroscopy (2DES), a powerful technique offering a unique combination of temporal and spectral resolution. The ultrabroadband pulses cover both the transverse and longitudinal nanorod resonances, enabling an accurate analysis of their distinct behavior. By complementing experiments with a quantitative model of hot‐carrier‐mediated nonlinearities that incorporates sample polydispersity, the broadband excitation, and the nanorods’ resonant absorption, the work provides a comprehensive understanding of the underlying mechanisms and identifies fingerprints of electron–electron scattering in the 2DES maps. Performed on a simple yet prototypical system, this analysis advances the study of plasmonic hot carriers and supports further applications of 2DES to explore ultrafast mechanisms in more advanced hybrid plasmon‐based systems, e.g. strongly‐coupled complexes.

Abstract Helical bilayer nanographenes (HBNGs) represent a promising class of materials for advanced optoelectronic and chiroptical applications, owing to their extended π ‐conjugation, intrinsic chirality, and tunable interlayer interactions. Here, the rational design and synthesis of bay ‐functionalized [7]HBNGs incorporating methoxy, diketone, and phenazine moieties is reported. These tailored modifications enable precise tuning of redox properties and significantly enhance photophysical properties, including pronounced spectral shifts, prolonged fluorescence lifetimes of up to 10 ns, and fluorescence quantum yields reaching 55%. Importantly, the resulting [7]HBNGs exhibit strong electronic circular dichroism and exceptional circularly polarized luminescence (CPL), with CPL brightness values as high as 110 M −1 cm −1 . The modular, convergent strategy enables versatile late‐stage modification of bay‐substituted HBNGs from a single scalable precursor, streamlining access to diverse derivatives and opening new opportunities for their application in molecular electronics, chiroptical devices, and advanced optoelectronic materials.

Abstract Dual‐band organic photodetectors (DB‐OPDs) offer adaptive detection of two distinct wavelengths by switching the voltage, making them promising candidates for wearable bio‐imagers. Although many studies have been conducted on rigid DB‐OPDs to date, flexible DB‐OPD with comparable performance to rigid counterparts has not been reported because of the vulnerability and lower transmittance of flexible substrates and electrodes. In this study, a 5.6‐µm‐thick ultra‐flexible DB‐OPD for visible and near‐infrared (NIR) light selective detection is reported. It shows high mechanical durability with stable electrical characteristics after 500 repetitions of intense bending at a radius of 0.5 mm. Furthermore, high specific detectivities exceeding 10 11 Jones in both the visible and NIR spectral ranges are achieved by incorporating an efficient donor to enhance the photocurrent and depositing a top electron transport layer to suppress the dark current, while minimizing damage to the underlying NIR‐sensitive layer. To validate the feasibility of the ultra‐flexible DB‐OPD in bio‐imaging, dual‐spectral peripheral oxygen saturation (SpO 2 ) measurements are performed under a continuous‐spectrum light source covering both the visible and NIR regions. The results highlight the potential of the ultra‐flexible DB‐OPD for wearable oximeter application.

Abstract Understanding the interplay between magnetic ordering and light emission is crucial for developing magneto‐optical technologies. However, this phenomenon is poorly understood since observations of this coupling vary significantly across materials. In this context, hybrid organic‐inorganic metal halide perovskites (HOIPs) that incorporate Mn 2+ ions are a chemically and structurally tunable platform for exploring this phenomenon, since they exhibit magnetic ordering and photoluminescence (PL) emission. Here, two antiferromagnetic Mn‐based HOIPs with different organic cations are studied, resulting in distinct lattice stiffness, Mn 2+ ‐Mn 2+ distance, and octahedral distortion. Temperature‐dependent PL excitation spectroscopy reveals changes in crystal field splitting energy and Racah parameters well above the Néel temperature (T N ), indicating the emergence of Mn 2+ ‐Mn 2+ magnetic interactions prior to reach long‐range magnetic ordering. These variations align with the observed changes in temperature‐PL evolution. The compound with a more rigid lattice shows stronger changes closer to T N , suggesting combined effects of magnetic polarons and spin‐canting. In contrast, magnetic modifications induced by magnetic polarons prevail in the HOIP with a softer lattice. These results reveal the complexity of the magneto‐optical coupling in Mn‐based HOIPs and provide new insights into this field extensible to other 2D materials that exhibit this phenomenon with potential for advanced magneto‐optical applications.

Abstract Two‐dimensional (2D) in‐plane heterostructures including compositionally graded alloys and lateral heterostructures with defined interfaces display rich optoelectronic properties and offer versatile platforms to explore one‐dimensional (1D) interface physics and many‐body interaction effects. Graded Mo x W 1 − x S 2 alloys show smooth spatial variations in composition and strain that continuously tune excitonic emission, while MoS 2 –WS 2 lateral heterostructures contain atomically sharp interfaces supporting 1D excitonic phenomena. These single‐layer systems combine tunable optical and electronic properties with potential for stable, high‐performance optoelectronic devices. Hyperspectral and nano‐resolved photoluminescence (PL) imaging enable spatial mapping of optical features along with local variations in composition, strain, and defects, but manual interpretation of such large datasets is slow and subjective. Here, a fast and scalable unsupervised machine‐learning (ML) framework is introduced to extract quantitative and interpretable information from hyperspectral PL datasets of graded Mo x W 1 − x S 2 alloys and MoS 2 –WS 2 heterostructures. Combining principal‐component analysis (PCA), t‐distributed stochastic neighbor embedding (t‐SNE), and density‐based spatial clustering of applications with noise (DBSCAN), spectrally distinct domains associated with composition, strain, and defect variations are uncovered. Decomposition of representative spectra reveals multiple emission species, including band‐edge excitons and defect‐related transitions, demonstrating that ML‐driven analysis provides a robust and automated route to interpret rich optical properties of 2D materials.

Abstract Efficient solar‐thermal conversion is crucial for applications including de‐icing, energy harvesting, and thermal regulation in outdoor environments. However, most existing photothermal coatings suffer from limited light absorption and poor mechanical durability, leading to performance degradation under cold and low‐irradiance conditions. Here, a durable ultra‐black superhydrophobic coating is reported and fabricated through a simple spraying process, in which carbon nanotubes (CNTs), titanium nitride nanoparticles (TiN NPs), and a fluorocarbon silane are incorporated into a polydimethylsiloxane (PDMS) matrix. The resulting hierarchical micro/nanostructure exhibits an exceptionally low reflectance of 0.66%, excellent water repellency, and strong anti‐icing capability. The micro/nanostructured surface morphology efficiently traps incident light, while the TiN and CNTs form a synergistic system where localized surface plasmon resonance (LSPR)‐induced near‐field enhancement significantly amplifies the photonic absorption, thereby improving broadband light harvesting and photothermal conversion. Under 1 sun irradiation, the coating rapidly heats to 70.1 °C, achieving efficient defrosting and de‐icing. Even at −10 °C under 0.3 sun, the temperature rise is fourfold higher than that of TiN‐free coatings. Moreover, TiN NPs enhance CNT dispersion and strengthen the filler‐matrix interface, yielding excellent durability. This work provides a simple and scalable strategy for multifunctional photothermal coatings with reliable performance in energy‐limited cold environments.

Abstract The design of advanced materials often reveals how apparent imperfections, such as structural defects or impurities, can be transformed into functional advantages. In insulating oxide matrices, the controlled introduction of dopant ions is the first step toward efficient photoluminescence. Later, the engineering of additional defects, often detrimental for photoluminescence, gives rise to unique capabilities for optical energy storage and persistent luminescence. Initially driven by biomedical applications, nanomaterials currently occupy a central role in persistent phosphor research. However, elaboration processes allowing to preserve their nanoscale usually involve poor control over their crystallinity, leading to performance behind that of bulk materials. Developing nanophosphors with well‐defined morphology and energy levels engineered for tailor‐made and efficient energy storage presents a significant materials challenge. Yet once again, what seems a limitation may prove to be a powerful opportunity. By exploiting the nanoscale to engineer energy storage in an unprecedented manner, persistent nanophosphors can open a new era in advanced optical materials. This perspective highlights how emerging applications, progress in nanoscale synthesis, surface engineering, and integration into advanced architectures are opening the path toward multifunctional, application‐ready materials. Altogether, the nanoscale offers a transformative avenue that can enable persistent nanophosphors to outperform their bulk counterparts.